glsl/linker: Recurse on struct fields when adding shader variables

glsl/linker: Recurse on struct fields when adding shader variables

ARB_program_interface_query requires that we add struct fields
recursively down to basic types.

Fixes 52 struct test cases in dEQP-GLES31.functional.program_interface_query.*

Signed-off-by: Kristian Høgsberg Kristensen <kristian.h.kristensen@intel.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>